More particularly, the invention relates to such a device comprising, on the one hand, switching cells each having two switches, each switch itself being constituted by at least one component forming a switch, and, on the other hand, capacitors associated with the switching cells and adapted to maintain between the homologous terminals of the two switches of each cell a load voltage equal to a fraction of the voltage of the voltage source, increasing as a function of its rank starting from the current source, and the homologous terminals of the switches situated at the end of the device close to the current source being “short-circuitable”, and furthermore control devices each connected to a switching cell and adapted to control the switchings of the two switches of the cell whilst ensuring opposite states for them, as well as means for operating the control devices adapted to supply a reference signal adapted to the desired conversion.
Such a device for reversible conversion of electrical energy is described in the European Patent published under the number EP 0 555 432.
It comprises N switching cells, N being any whole number greater than or equal to 2. Each cell is composed of two switches which are controlled to have complementary states at each instant. N switches of the N cells are connected in series and constitute a first series of the device, the N other switches being connected in series and forming a second series of the device.
The two series of switches are interconnected, on the one hand, by a common extremity to a current source and, on the other hand, by their opposite extremities to the terminals of a voltage source.
A capacitor which is connected between the symmetrical terminals of the two switches of the cell concerned is associated with each switching cell. The cell closest to the voltage source may be associated with a specific capacitor in the hypothetical case in which the voltage source is not an ideal source in order to compensate for these imperfections.
In the contrary case, the perfect voltage source plays the role of capacitor with regard to this cell.
Each capacitor has as its function to maintain a voltage known as a capacitor load voltage at its terminals.
A distribution of these load voltages proportional to the rank k of each capacitor,             Vc      k        =          kV      N        ,V being the voltage at the terminals of the voltage source, assures at the terminals of the blocked switches a voltage difference equal to   V  Nfor all the blocked switches. Thus each capacitor is chosen so as to present a behaviour under voltage as an increasing function of its rank, greater than the value       kV    N    .
In addition, control logics can be synchronised in such a way that the ripple of the output voltage of the device has an amplitude equal to   V  Nand a frequency equal to NF, where F is the switching frequency of the switching cells.
This output voltage is the voltage between the terminal of the voltage source situated at the lowest potential and the terminal of the current source connected to the conversion device.
However, we are witnessing nowadays the development of high-powered electrical energy conversion devices for increasingly high voltage levels of the voltage source. This voltage increase leads indirectly to an increase in the size of the capacitors of the device which have to support increasingly substantial fractions of this voltage. Also nowadays, above 6 kV the price and the volume of the device tend to become prohibitive.